A hydraulic braking system for an automotive vehicle typically includes a master cylinder, that is coupled with a brake operating member such as brake pedal, via a power boosting device such as vacuum booster or hydraulic booster, or alternatively directly without such a power boosting device interposed, so as to produce a master cylinder pressure corresponding to an operating force imposed on the brake operating member. There are two cases in one of which the master cylinder pressure is normally applied to a brake cylinder to operate a brake, and in the other of which a hydraulic pressure of a power-operated hydraulic pressure source provided separately from the master cylinder is normally supplied to the brake cylinder by being controlled by a hydraulic pressure control valve device, and in the event of a failure of the power-operated hydraulic pressure source or the hydraulic pressure control valve device, the master cylinder pressure is applied to the brake cylinder.
In either case, when the brake cylinder is operated by supply of a working fluid or a brake fluid from the master cylinder, a substantial amount of the brake fluid is required to be supplied before a braking effect actually starts to be given, since the brake fluid is consumed for extinguishing a brake clearance between a friction member and a brake rotor, or between the friction member and a pressing member that presses the friction member against the brake rotor, and for elastically deforming a relatively soft member such as sealing member. Such a supply of the brake fluid is called “fill-up”, and it is conventional to equip the master cylinder with fill-up function.
More specifically, as disclosed in Patent Document 1 set forth below, for instance, a fill-up chamber is provided in the master cylinder, in addition to a pressure chamber the brake fluid in which is pressurized. The fill-up chamber is usually connected to the pressure chamber through a communication passage in which a check valve is disposed, and also to a low pressure source such as reservoir through a parallel circuit of a relief valve and a restricting portion. During a fluid pressure of the fill-up chamber is not larger than a relief pressure of the relief valve, the brake fluid of the fill-up chamber is supplied to the pressure chamber via the communication passage and the check valve, thereby implementing the “fill-up”. With an increase in the fluid pressure of the pressure chamber (i.e., a master cylinder pressure), the fluid pressure of the fill-up chamber also increases to eventually exceeds the relief pressure, thereby opening the relief valve to allow the brake fluid to flow from the fill-up chamber to the low pressure source, whereat the fill-up terminates. In order to efficiently utilize a brake operating force in raising the master cylinder pressure, it is desirable that the fluid pressure of the fill-up chamber lowers down to the atmospheric pressure. Hence, the fill-up chamber is desirably in communication with the low pressure source by bypassing the relief valve. However, since during the fill-up it is necessary to raise the fluid pressure of the fill-up chamber beyond the relief pressure, a restricting portion is disposed in parallel with the relief valve.
Patent Document 1: JP-A-2002-211383